1. Field of the Invention
The present invention relates to a short-wavelength laser light source and an optical information processing apparatus used for optical information processing, optics-applied measurement control, or optical communication where coherent light is used.
2. Description of the Related Art
In the field of optical information processing, a 10 mW or larger output power is required for the short-wavelength laser light source used for optical recording. A combination of a semiconductor laser and a light-wavelength converting device capable of generating a harmonic of the laser light (i.e., an optical frequency doubler) shows promise as a blue laser light source. FIG. 13 shows the construction of a blue-light emitting, short-wavelength laser light source of the related art. A fundamental wave P1 emitted from a semiconductor laser 21 is first collimated by a collimator lens 24, and then focused by a focusing lens 25 onto an optical waveguide 2 formed in a light-wavelength converting device 22. The fundamental wave is passed through the optical waveguide 2 for conversion into a harmonic wave P2 which is output. Part of the output harmonic wave P2 is deflected 90 degrees by a beam splitter 26 and detected by a detector 27, by which the semiconductor laser 21 is controlled in order to maintain a constant output. In this manner, a constant harmonic wave P2 is output.
Next, the light-wavelength converting device used in the above arrangement will be described in detail. FIG. 14 shows the construction of the light-wavelength converting device of the related art. The following describes in detail the generation of a harmonic wave (wavelength 437 nm) from a fundamental wave of 873-nm wavelength with reference to the accompanying drawings. (Refer to Kazuhisa Yamamoto and Kiminori Mizuuchi, "Blue light generation by frequency doubling of a laser diode in a periodically-domain inverted LiTaO.sub.3 waveguide," IEEE Photonics Technology Letters, Vol. 4, No. 5, pp. 435-437, 1992.) As shown in FIG. 14, the optical waveguide 2 is formed in an LiTaO.sub.3 substrate 1, and layers 3 with periodic inversion of polarization (polarization inversion layers) are formed in the optical waveguide 2. The polarization inversion layers (i.e., domain-inverted regions) 3 are periodically interleaved with polarization non-inversion layers 4 to compensate for the propagation constant mismatch between the fundamental wave and the generated harmonic wave so as to increase the efficiency of harmonic generation. First, the principle of harmonic amplification in the light-wavelength converting device will be described with reference to FIG. 15. In a polarization non-inversion device 31 with no polarization inversion, no polarization inversion layers are formed and the polarizations are oriented in the same direction. In this polarization non-inversion device 31, a harmonic output 31a just repeats increasing and decreasing along the direction of propagation through the optical waveguide. On the other hand, in the case of a polarization inversion wavelength converting device (first-order cycle) 32 with periodic inversion of polarization, the harmonic output, designated as an output 32a in FIG. 15, increases with the square of the length L of the optical waveguide as shown. However, in the polarization inversion device the harmonic output P2 is obtained from the fundamental input P1 only when pseudo phase matching is achieved. This pseudo phase matching is achieved only when the interval .LAMBDA.1 of the polarization inversion layers coincides with .lambda./(2(N2.omega.-N.omega.)), where N.omega. is the effective refractive index for the fundamental wave (wavelength .lambda.) and N2.omega. is the effective refractive index for the harmonic wave (wavelength .lambda./2). In this manner, the light-wavelength converting device of the related art is based on the polarization inversion structure.
A fabrication method for this device will be described below. A pattern of Ta repeating at intervals of a few micrometers in width is formed by deposition and photolithography on the LiTaO.sub.3 substrate 1, a nonlinear optical crystal. Next, proton exchange is performed at a temperature of 260.degree. C., which is followed by heat treatment at about 550.degree. C. to form polarization inversion layers 3 of opposite polarization to that of the LiTaO.sub.3 substrate 1. A slit is formed again from Ta, after which heat treatment is performed with pyrophosphorous acid (260.degree. C.) for 12 minutes, followed by annealing at 420.degree. C. for one minute. The result is the formation of the optical waveguide 2. With the thus fabricated light-wavelength converting device, a power of 1.1 mW was obtained for the harmonic wave P2 generated from the fundamental wave P1 of 873-nm wavelength when the length of the optical waveguide was set at 10 mm and the power of the fundamental wave P1 at 37 mW. Furthermore, the above light-wavelength converting device had an allowable width of as narrow as 0.1 nm for the fundamental wave, and did not allow mode hopping or wavelength spread of the semiconductor laser.
In the short-wavelength laser light source based on the above semiconductor laser, operation with a power higher than 100 mW is difficult when the reliability of the semiconductor laser is considered; furthermore, when the lens loss and the optical waveguide coupling loss are considered, the available fundamental output is about 50 to 70 mW. Consequently, the harmonic output that can be obtained is only 2 to 4 mW, and therefore, it is difficult with the above device to obtain a consistently harmonic output of over 10 mW which is the practical level of a short-wavelength laser in the field of optical information processing.